Radar measuring devices determine the fill level of a fill substance in a container via the travel time of measuring signals. Travel-time methods utilize the physical law that the travel distance equals the product of travel time and propagation velocity of the measuring signals. In the case of fill level measurement, the travel distance is twice the separation between the antenna and the surface of the fill substance. If the measuring signals are microwaves, then the wanted echo signal, thus the part of the measuring signals reflected on the surface of the fill substance, and its travel time are determined based on the so-called echo function, respectively the digital envelope curve. The envelope curve plots the amplitudes of the echo signals as a function of the separation ‘antenna-surface of the fill substance’. The fill level can then be determined from the difference between the known separation of the antenna from the floor of the container and the separation of the surface of the fill substance from the antenna, as determined from the measuring.
In the case of microwaves measuring devices, one distinguishes between measuring devices, which work according to the pulse radar method and utilize broadband high-frequency pulses, and FMCW measuring devices (Frequency Modulated Continuous Wave), in the case of which continuous microwaves of wavelength λ are periodically linearly frequency modulated, e.g. with a sawtooth voltage.
In order to assure a constantly good measuring performance, the antenna elements radiating and receiving the measuring signals and the measuring electronics are sealed on the process-side with process isolation elements against external influences. Depending on location of use, the fill level measuring devices may experience high temperatures, high pressures and/or aggressive chemical media. As a function of conditions reigning at the measuring location, requirements placed on protection of the sensitive electronics components are correspondingly high.
Known from published international patent application WO 2006/120124 A1 is a fill-level measuring device, in the case of which the horn antenna is at least partially filled with a temperature-stable, dielectric material. The dielectric material is so dimensioned that, at normal temperature, a defined separation between the outer surface of the dielectric filler body and the inner surface of the adjoining antenna element is present. Due to such embodiment, the filler body can expand with increasing temperature, so that no mechanical stresses occur within the filler body.
In the case of radar measuring devices sold by the applicant under the mark, MICROPILOT, the process isolation is usually manufactured of PTFE. PTFE has the advantage that it is almost transparent for microwaves. Additionally, it has a temperature-, pressure- and/or chemical durability sufficient for a large number of applications in process automation. Moreover, PTFE has the advantage that it makes accretion formation difficult. If accretion formation does nevertheless occur on the process isolation, then cleaning it away is quite easy.
The process isolations are preferably isostatically pressed and subsequently brought to final shape by mechanical processing. The isostatically pressed components are distinguished by a high homogeneity, whereby sensor downtimes and measurement error—caused by inhomogeneities in the region of the process isolation—can be excluded.
In process isostatic pressing, the physical effect is utilized that pressure propagates in liquids and gases in all directions uniformly and produces on areas exposed to it forces, which are directly proportional to such areas. In the case of PTFE processing, a rubber mold is filled with plastic powder or granular plastic, especially with PTFE powder or PTFE granules, and then brought liquid tightly closed into the pressure vessel of a pressing apparatus. The pressure, which acts via the liquid in the pressure vessel on all sides of the rubber mold, compresses the enclosed PTFE powder uniformly. After the pressing, the PTFE molded part is removed from the rubber mold and sintered in a sinter oven.
A disadvantage of isostatically pressed PTFE process isolations is that PTFE is relatively expensive. PP powder, respectively PP granular material, is less expensive than PTFE granular material, respectively PTFE powder, by about a factor of 10.